Moscow’s military force in Syria has grown to about 4,000 personnel, but this and more than a month of Russian air strikes have not led to pro-government forces making significant territorial gains, U.S. security officials and independent experts said.

Moscow, which has maintained a military presence in Syria for decades as an ally of the ruling Assad family, had an estimated 2,000 personnel in the country when it began air strikes on Sept. 30. The Russian force has since roughly doubled and the number of bases it is using has grown, U.S. security officials said.

The Russians have suffered combat casualties, including deaths, said three U.S. security officials familiar with U.S. intelligence reporting, adding that they did not know the exact numbers. […]